1. Field of the Invention
The present invention relates generally to communication devices. More specifically, duplex communication methods and systems with maximum gain limiting such as in telephone handsets and speaker phones are disclosed.
2. Description of Related Art
A telephone transmitter (microphone) into which a person talks converts speech, i.e., acoustical energy, into variations in an electric current, i.e., electrical energy, by varying or modulating the loop current in accordance with the speech of the talker. On the other hand, telephone receiver (speaker) converts electrical current variations into sound that the person can hear.
A small amount of the transmitter signal is fed back into the receiver, called the sidetone, as a form of feedback mechanism. In other words, sidetone is the portion of the talker's voice that is fed back to the talker's receiver. Sidetone allows the person to hear his/her own voice through the receiver to help the person determine how loudly to speak. Sidetone should be at a proper level because too much sidetone may cause the person to speak too softly for good reception by the other party while too little sidetone may cause the person to speak too loudly that it may sound like a yell at the receiving end.
Receiver gain may be applied to facilitate the user in hearing the receiver output. For example, amplified telephones for hearing impaired users and speakerphones often employ receiver gain to facilitate the user in hearing the receiver output. Although various anti-sidetone circuits are often employed to minimize feedback and to reduce transmission of acoustical noise, in general no more than 30 dB of receiver gain (over nominal) can be provided because of feedback between the transmitter (microphone) and the receiver (speaker) that result in howling. This howling is similar to when a microphone is placed too close to a public address system speaker. However, 30 dB of gain is generally insufficient to allow the a user with moderate to severe hearing loss to adequately hear the receiver output on the telephone. Other amplified telephone sets may employ a line echo canceller (LEC) and an acoustic echo canceller (AEC) in order to achieve gains of more than 30 dB. However, the maximum amount of gain that can be achieved without unstable operation or howling may vary depending on the actual conditions under which the telephone set is utilized, e.g., the local loop to which the telephone set is connected, etc. Conventional amplified telephone sets are generally designed to take into account the worst case scenario and limits the maximum amount of gain applied by the telephone set accordingly. Thus, the maximum amount of gain that the amplified telephone set can apply is often be lower than what can actually be applied while preventing unstable operation or howling.
Thus it would be desirable to maximize or increase the receiver gain without generating feedback that results in howling.